How does the scandal referred to as ""The Doctors' Plot"" contribute to the author's characterization of Stalin?

It proves he was racist against Jewish people.

Explanation:

For Stalin, whose deeds easily matched those of Hitler and whose deceits had been evident throughout his life, the Doctors' Plot and intended show trial were meant to cleanse the Soviet Union of "foreign," "cosmopolitan," and "Zionist" (read Jewish) elements.